Generally, the construction of a manhole includes an underground horizontal main sewer line having vertical access pipes or sections connected to the main sewer line which lead to a truncated cone and ultimately to a manhole frame and cover. On top of the cone a manhole chimney or adjustment courses are constructed which lead upward and are of the same diameter as the upper surface of the cone. The manhole frame is placed on top of this chimney and holds the manhole cover.
Past constructions of the manhole chimney or adjustment courses included using brick and mortar to build up the manhole chimney or in the alternative precast concrete rings have been used which are stacked on top of one another. The state of the art is generally shown in the following cited references: U.S. Pat. No. 1,639,495 issued to Frame; U.S. Pat. No. 3,926,533 issued to Binette; German Pat. No. 2,525,285; U.S. Pat. No. 4,121,390 issued to Hall; U.K. Pat. No. 2,088,446; U.K. Pat. No. 2,102,479, U.S. Pat. No. 4,408,421 issued to Pai; and U.S. Pat. No. 4,469,467 issued to Odill, et al.
Upon completion of the construction, the surrounding surface is back filled, and the upper surface is paved such that the manhole is generally flat with the surrounding roadway surface. In areas other than streets the top of the cover is also generally adjusted to the surrounding ground level.
Because of the numerous and varying stresses on these constructions, manhole chimney constructions of the past have been prone to rapid deterioration. For instance, the hydrogen sulfide gas and other chemicals commonly found in sewer systems, may be corrosive to cement and may tend to deteriorate the preformed rings or the cement which holds the brick constructions together. In addition, the freeze-thaw cycles of the surrounding ground place strong pressure on the construction and may crush the construction. These freeze-thaw cycles may also work to widen any cracks or deformations in the structure thereby breaking down the structure. Also jolts or shock waves produced by passing traffic and road scrapers accentuate the destructive forces acting on the construction thereby breaking down the construction even sooner. Because of these and other problems the integrity of the structure is eventually broken such that water and sediment may be permitted to flow through the manhole chimney and into the sewer system.
This process is called infiltration and is undesireable in a sewer system in that these sediments may cause clogging of the sewer system or create undesireable conditions for workers working inside the sewer system. In addition, infiltration may soon result in a void in the underlying support of the roadway surface were the sediment has entered through the breaches in the manhole chimney, which may then cause structural problems with the roadway surface since it is not supported where the void occurs.
The precast spacing rings of the past have also been troublesome to install in a manhole chimney. These precast rings are heavy and require heavy equipment or several men to install. In addition these rings tend to be somewhat fragile and may shatter or crack if dropped or mishandled.
In the past, constructions have been attempted wherein seals have been placed between the precast ring in an attempt to stop the infiltration problem. In addition, as disclosed in U.S. Pat. No. 4,469,467 issued to Odill and assigned to Cretex Companies, manhole chimney seals have been attempted which seal the manhole chimney by a rubber tubular seal placed either on the inside or the outside of the manhole chimney. However, these constructions may still allow freeze-thaw damage or other damage to the chimney construction to occur.